Order by decode statement with Union all

Question

Hi
I use Database Version: Oracle 9.2.0.1.0
OCI version 9.2
I try to use order by decode while i have a select base on union and I get en error:
ORA-01785: ORDER BY item must be the number of a SELECT-list expression
Here is my code:
select catalog_type, catalog_id, item_id, item_name
from items_catalog
union all
select catalog_type, catalog_id, organisation_unit_id, organisation_unit_name
from units_catalog
order by decode(catalog_type, 1, item_id, organisation_unit_id)
If I use the decode on a select statement without union, it works.
What I have to do in order to make it work on union all select?
Tnx

Answer 1

Hi,
Here's a slightly different way:
COLUMN     sort_key   NOPRINT
VARIABLE     catalog_type_wanted     VARCHAR2 (5)
EXEC         :catalog_type_wanted := '4'; 
SELECT       catalog_id, item_id, item_name
,       CASE
           WHEN  :catalog_type_wanted = '1'  THEN  '1'
                                           ELSE  '2'
       END || item_id          AS sort_key
FROM       items_catalog
      UNION ALL
SELECT    catalog_id, organisation_unit_id, organisation_unit_name
,       CASE
           WHEN  :catalog_type_wanted = '1'  THEN  '2'
                                           ELSE  '1'
       END || organisation_unit_name     AS sort_key
FROM       units_catalog
ORDER BY  sort_key
;This way guarantees that (for example) 'SHOP50' comes before 'SHOP58', whether or not they are in the first half of the output or the second half.
Notice that this uses the SQL*Plus COLUMN ... NOPRINT command to hide the sort_key column.
If you're not using SQL*Plus, then whatever front end you are using may have a similar feature.
If not, then you can do the UNION in a sub-query, as Daniel did, but do the ORDER BY in the main query (only).